Combination Effects of Plant Extracts Rich in Tannins and Saponins as Feed Additives for Mitigating in Vitro Ruminal Methane and Ammonia Formation

SIMPLE SUMMARY: Ruminant livestock contribute to global warming by emitting methane, a major greenhouse gas, as a product of microbial fermentation occurring in the rumen. Apart from its contribution to greenhouse gas emissions, methane emissions represent an energy loss in ruminants. Excessive ruminal ammonia formation, on the other hand, leads to a higher risk of pollution via ammonia, nitrous oxide, and nitrate emissions. Natural plant secondary compounds such as tannins, saponins, and essential oils are among the promising feed additives to mitigate enteric methane and ammonia formation. Though both tannins and saponins, when tested separately, have been reported to be effective, their combinations have rarely been tested. Therefore, in the present study, whether the combination of plant extracts rich in tannins and saponins would act additively or non-additively (associatively) in decreasing methane and ammonia formation in an artificial rumen system was investigated. Indeed, the addition of plant extracts rich in tannins and saponins, either individually or in combination, decreased the methane proportion of total gas in both high-forage and high-concentrate diets. This indicates their effectiveness as anti-methanogenic agents across contrasting diet types. Their effects were generally additive and occasionally synergistic (i.e., more than proportionate), especially in mitigating ruminal ammonia formation and, less clearly, concerning methane emissions. ABSTRACT: The objective of this experiment was to test the effects of combining plant extracts rich in tannins and saponins at varying proportions on in vitro ruminal methane and ammonia formation. Tannins were extracted from Swietenia mahogani leaves and saponins from Sapindus rarak fruits with various solvents. The extracts obtained with the most efficient solvents (tannins: 75% water and 25% methanol; saponins: pure methanol) were then used in vitro. The treatments consisted of two substrate types (high-forage (HF) or high-concentrate (HC) diets) and five extract combinations (tannins: saponins, 1:0, 3:1, 1:1, 1:3, and 0:1) added at 2 mg/mL in incubation liquid. In vitro incubation was performed in four runs, with each treatment being represented with two replicates per run. The addition of plant extracts rich in tannins and saponins, either individually or in combination, decreased the methane proportion of total gas in both the HF (p < 0.05) and HC (p < 0.05) diets. The effects of the plant extracts rich in tannins and saponins were generally additive in mitigating methane emissions. Favorable associative effects between the extracts were observed in the ammonia concentration, both in the HF (p < 0.001) and HC (p < 0.01) diets and in the methane proportion of total gas, with a 1:3 mixture of tannins and saponins added to the HC diet (p < 0.05).